Rubber compositions for tires comprise, in a known way, plasticising agents used for the preparation or synthesis of certain diene elastomers, for improving the processability of said compositions in the raw state and some of their use properties in the cured state, such as, for example, in the case of tire treads, their grip on wet ground or also their abrasion and cut resistance.
For a very long time, it has essentially been oils derived from petroleum and which are highly aromatic, known under the name of DAE (“Distillate Aromatic Extracts”) oils, which have been used to perform this function of plasticising agent. Many tire manufacturers are today, for environmental reasons, envisaging gradually replacing these DAE oils by substitute oils of the “non-aromatic” type, in particular by oils known as “MES” (“Medium Extracted Solvates”) or “TDAE” (“Treated Distillate Aromatic Extracts”) which are characterized by a very low level of polyaromatics (approximately 20 to 50 times less).
The Applicants have noted that the replacement, in tire rubber compositions, of DAE aromatic oils by these MES or TDAE oils is unexpectedly reflected by a reduction in the abrasion and cut resistance of said compositions, it being possible for this reduction to be even totally unacceptable in certain applications, in particular with regard to the problem of chipping of tire treads.
“Chipping” (or “scaling”) is a known damaging mechanism which corresponds to lamellar surface pieces, in the form of scales, being torn off from the constituent “rubber” (or rubber composition) of the treads under certain aggressive running conditions. This problem is encountered in particular with regard to tires for off-road vehicles or vehicles found on construction or civil engineering sites, which have to run over different types of soils, some of them stony and relatively aggressive; it has, for example, been described, along with some solutions for overcoming it, in the patent documents EP-A-0 030 579, FR-A-2 080 661 (or GB-A-1 343 487), or also in U.S. Pat. No. 3,927,144 and U.S. Pat. No. 4,968,764, which recommend, for overcoming it, the use of cyclopentadiene-based hydrocarbon resins.
On continuing their research, the Applicants have discovered that the replacement of a portion of these MES or TDAE oils by another specific plasticising agent makes it possible not only to solve the above problem but, which is even more surprising, to also improve the abrasion and cut resistance of the rubber compositions using conventional aromatic oils as plasticising agent.